Sheets for automobile bodies must have high formability and strength; hence, cold-rolled steel sheets have been used for such automobile bodies. However, in order to achieve high fuel efficiency and in order to achieve weight reduction, rolled aluminum alloy sheets have been recently used. In particular, Al—Mg—Si alloy sheets are suitable for automobile bodies. This is because these alloy sheets, which have not yet been subjected to aging heat treatment, are softer and have higher formabilities such as bendability as compared with other materials. Furthermore, the alloy sheets can be increased in strength by heating the alloy sheets during a bake-painting step or another step subsequent to a forming step.
For the Al—Mg—Si alloy sheets, the following attempt has been being made: an attempt to enhance the formability by controlling the size and/or state of intermetallic compounds and/or precipitates. Furthermore, the following attempt has been being made: an attempt to enhance the bake hardenability and the formability, for example, the bendability, by appropriately tuning the composition and performing appropriate heat treatment in processes for manufacturing such alloy sheets. For example, Japanese Unexamined Patent Application Publication No. 9-31616 discloses the following technique: in order to control the size and/or state of intermetallic compounds and/or precipitates, the total Mg and Si content is kept at 2.4% or less, at least one selected from the group consisting of Mn, Cr, Zr and V is used to refine grains and stabilize microstructure, and a cast slab is homogenized, hot-rolled, cold-rolled, and subjected to solution heat treatment.
In known techniques disclosed in Japanese Unexamined Patent Application Publication No. 9-31616 and other documents, at least one selected from the group consisting of Mn, Cr, Zr and V is used to refine grains and to stabilize microstructure and a finished sheet is evaluated for the precipitation state of intermetallic compounds, stretchability, bendability, and the like. In general, alloy sheets with a total Mg and Si content of 1.5% or less have unsatisfactory bake hardenability. For such alloy sheets, the following items have not been sufficiently investigated: the influences of Mg and Si on the bake hardenability and the influence of Cr on the surface quality (orange peel), the bendability, and the size of recrystallized grains of a finished sheet. In order to enhance the bake hardenability, bendability, and surface quality (orange peel) of an aluminum alloy sheet to be processed into a finished sheet, there is a problem in that manufacturing cost is high because a step of manufacturing a slab by a DC casting process is necessary and a large number of the following steps are also necessary according to needs: a scalping step, a homogenizing step, a hot-rolling step, a cold-rolling step, an intermediate annealing step, a final-rolling step, and a final-annealing step.